Dental rinses are well known in the art of dental hygiene. Such rinses can be formulated for several different purposes, for example, the retardation of tooth decay by the addition of fluoride, the loosening or removal of plaque, the removal of mouth odor, and the reduction of the level of various bacteria resident in the oral cavity. Some of these desirable characteristics can be combined in a single rinse, some cannot be combined, or cannot be combined without secondary problems because of interaction of the various components which are required to carry out the various desired activities.
A highly successful and accepted dental rinse for the loosening of plaque, which renders it more amenable to removal during brushing with a conventional dentifrice is disclosed in U.S. Pat. No. 4,657,758 to Goldemberg, et al., the disclosure of which is incorporated herein by reference. This patent discusses a commercial product known by the trademark of Plax (manufactured by Oral Research Laboratories). The Plax rinse comprises, among other components well recognized in the art, an oral surfactant, preferably sodium lauryl sulfate, sodium benzoate which has been found to be effective in increasing the amenability of plaque to removal and an effective amount of detergent builder. The entire composition, which is sold in a dilution level at which it may be readily used by the purchaser is formulated to have a pH of from about 7.5 to about 10, preferably about 9.0. In order for the composition to be effective, the presence of a detergent builder is indicated as being essential. Furthermore, in order for the solution to be shelf stable, the pH must be held at the levels indicated, to prevent hydrolysis of the sodium lauryl sulfate. Goldemberg indicates that the presence of sodium benzoate increases the effectiveness of the composition. Goldemberg further indicates that sodium benzoate might have some anti-microbial properties. The literature however, indicates that such anti-microbial properties would be quite minimal at the pH levels of Plax.
Sodium benzoate per se has negligible anti-microbial properties. Benzoic acid however, has anti-microbial properties and these increase inversely with the degree of disassociation thereof. Clearly, sodium benzoate is substantially totally disassociated. The degree of disassociation of benzoic acid can be set forth as follows:
______________________________________ pH* 2 3 4 4.5 5 6 7 Undisassociated Acid 99 94 60 30 13 15 0.15 ______________________________________ *Manufacturing Chemist & Aerosol News Oct. 1969 p. 35
Since, as stated, the anti-microbial activity of benzoic acid is directly proportional to the amount of undisassociated molecule in the solution, it is clear that at the pH's listed by Goldemberg the anti-microbial activity due to sodium benzoate is negligible. This is impliedly acknowledged by Goldemberg who utilizes other and additional anti-microbial agents to attempt to achieve this effect.
Regrettably, it is not possible to merely lower the pH of the Plax composition to the minimum level for effectiveness of the benzoate moiety, that is to say, 5.5 or lower because such solutions would not be shelf stable.
It would therefore be desirable to formulate a solution which achieves the plaque loosening and removal properties of Plax, while having substantially increased anti-microbial activity.